A&G box coordinate system

  In theory, the A&G box coordinate system could be misaligned with the slit, although such an effect has not been observed until now. To check the alignment proceed as follows:

1. Set the turntable to a known position angle with respect to the slit.
2. Move the guide probe (and/or TV - both systems should be checked) to centre on stars of accurately known position (e.g. SA57 or Pleiades field) whilst guiding on the other probe.
3. Record all the star positions in guide probe coordinates and go away and do an astrometric analysis to get the scale, rotation and any field distortions.

NB: at present none of this is catered for in the control software, but at least a rotation between the A&G box and the detector should be allowed for so that Cassegrain imaging can be set up correctly.